Sometimes a book exists in more than one language, probably the author's native language and english. In such a case, the decision has to be made, which version of the document to convert for further use.
In some cases the answer is given by the fact which version of the document is more up to date. In any other case, the language should be used that the intended user community is most likely to accept preferrably. However, if the audience is international, english is the language of choice.